Mobile home furnace (C) Daniel FriedmanCombustion Air Requirements
How to Recognize, Test, & Diagnose Heating Appliance Combustion Air Problems

  • COMBUSTION AIR - CONTENTS: Combustion air requirements & combustion air defects at heating boilers, furnaces, water heaters. Diagnose signs of inadequate combustion air for an oil or gas burner. Dangers of carbon monoxide poisoning if there is soot production at gas fired heating appliances. Inadequate combustion air can cause dangerous carbon monoxide gas in buildings. Combustion air safety check procedure for gas fired heating equipment. How to test for safe combustion air for gas fired heating appliances, water heaters, etc. Combustion Air Defects & Safety Hazards at Mobile Home Heating System. How is outside combustion air provided to a woodstove?
  • POST a QUESTION or READ FAQs about combustion air requirements and safety hazards for oil and gas fired heating appliances

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Combustion air defects & hazards:

This article explains how to recognize and fix combustion air defects on heating appliances such as boilers, furnaces, and water heaters.

Lack of adequate combustion air causes improper heater operation, increased maintenance cost, and risks dangerous production of carbon monoxide gas. This article series explains how to recognize & diagnose problems with residential heating boilers, including loss of heat, heating boiler noises, leaks, odors, or smoke, and high heating costs.

This article series answers most questions about central hot water heating system troubleshooting, inspection, diagnosis, and repairs. Our photo at page top shows an oil fired furnace installed in a closet with an airtight door; there was no outside combustion air supply. The heating system could not work properly nor safely in this home.

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Visible Signs of Inadequate Combustion Air in Buildings

How to Recognize & Diagnose Inadequate Combustion Air, Sooting, or Burn Marks at Oil-Fired or Gas-Fired Heating Systems

Backpressure Burn at an oil burner (C) Daniel Friedman Backpressure sooting at an oil fired furnace (C) Daniel Friedman

The photographs above show a heating furnace with soot blow-back around the oil burner probably means there is backpressure in the combustion chamber - an improper operating condition that may be unsafe.

Watch out: inadequate combustion air supply to a gas burner (and less often to an oil burner) is very dangerous and can produce potentially fatal carbon monoxide. If you suspect unsafe heating system operation or a carbon monoxide problem be sure everyone leaves the building immediately and then call your local fire department for assistance.

Clues Indicating Possible Lack of Combustion Air & Related Safety Hazards

Mobile home furnace (C) Daniel FriedmanLack of adequate combustion air can be indicated by or can result in these heating system operating and safety worries:

  • Watch out: Gas burner sooting or odors: SAFETY WARNING: Small amounts of soot or flame marks right at the gas burner also indicate an operating problem but may not be producing carbon monoxide. But soot produced at a gas fired appliance such as chunks of soot found around a gas flue vent or draft hood is a RED DANGER FLAG as dangerous carbon monoxide may be produced and a chimney may be blocked.

    Turn off the equipment and contact your heating service company or utility company immediately.
  • Improper oil burner system operation such as noises, rumbling, etc. discussed
  • Oil burner sooting or dirty operation (the photo at above-left)
  • Burn marks on the boiler (photo at above right), furnace, or water heater, (these conditions may be caused by a blocked exhaust flue and inadequate venting).

    Watch out: burn marks on a heating boiler or furnace (shown in our page top photo) can also be due to collapse or damage to the combustion chamber liner - a serious fire hazard needing immediate attention. Notice that in our photo at right, the brown burn marks around the oil boiler combustion chamber inspection port (that rusty round door above the oil burner) have been cleaned-off.

    The presence of these burn marks does not necessarily mean that the oil burner backpressure problem remains - in this case the system has been cleaned and adjusted, but no one has re-painted the front of the boiler. But if you see fresh peeling paint or soot in such an area further investigation is needed.

    That's why we recommend that after repairing a back-pressure problem at heating equipment the service technician should clean the boiler or furnace exterior - to remove confusing debris and to make it easier to see if the problem recurs.
  • Heating equipment located in a small utility room with no provision for combustion air intake. When the service technician adjusts the system she probably worked with the utility room door open, but when the service tech left the job he may have closed the door - completely changing the availability of combustion air for the equipment. We need about one square inch of un-louvered (unobstructed) combustion air intake per 1000 btuh of the oil fired heating boiler, furnace, or water heater.
  • Increased heating system operating cost, spending more on heating oil than necessary
  • Damage to oil burner components (backpressure heat can destroy an ignition transformer),
  • Higher and more frequent heating service calls & costs
  • Loss of heat, the heating system goes off on safety reset
  • Noises in the heating equipment or chimney when the boiler, furnace, or water heater starts, is operating, or is shutting down
  • Smoke or soot indoors,coming out of the heating equipment or its draft regulator
  • Carbon monoxide or CO alarms: Potentially, the production of carbon monoxide or other flue or combustion gases which escape into the building - potentially dangerous or even fatal. It is harder to produce with oil heat than with gas heat, but not impossible.

Also see UNSAFE HVAC DUCT OPENINGS which describes the risks of reduced combustion air on hot air heating systems when certain return air duct defects are present, and also see CARBON MONOXIDE - CO and CHIMNEY INSPECTION DIAGNOSIS REPAIR

Heating Equipment Combustion Air Rules of Thumb

Square Inches of Combustion Air Intake: 1 sq. in. per 1000 BTUH

For heating equipment installed in confined spaces, an old-timer's rule of thumb is to add up the total INPUT BTUH numbers from all of the data tags on all of the heating equipment installed. You want to see at least one of open fresh air intake per 1000 btuh.

Total Sq.In. of Combustion Air Intake Opening = Total Input BTUH / 1000

Watch out: this combustion air rule of thumb needs to be adjusted to account for the air flow restriction caused by louvers and screening over the combustion air intake opening. As a rule of thumb we

Reduce the effective total square inches of combustion air intake opening by at least 1/3 for louvers & screens

Reduce the effective total square inches of combustion air opening further if the screen is or can easily become clogged by lint, leaves, trash & debris

Watch out: The combustion air estimate provided by outside combustion air openings or openings into other, presumably larger building areas (see below) also needs to account for the effects of building exhaust fans, tight buildings, and similar interferences. (BACKDRAFTING HEATING EQUIPMENT)

Watch out: this rule of thumb falls apart if the fresh air is not being vented directly into the heating equipment area through an outside wall. That is, if air has to move through vent pipes or ductwork into the area where it is needed, the equivalent square inches of fresh air intake venting may need to be increased depending on the length, number of bends, angles of bends, and diameter of the fresh air or combustion air intake venting system.

Standard engineering approaches to calculating air flow through round or rectangular ductwork can solve the question of impact on combustion air of routing it through ducts.

Cubic Feet of Room Space as a Measure of Adequacy of Combustion Air: Total Input BTUH / 1000 x 50

For heating equipment installed in larger spaces, a common rule of thumb for computing the required total cubic feet of free space to assure adequate combustion air is to provide 50 cubic feet of free space per 1000 Input BTUH for the total of all of the heating appliances installed in the area. The assumption behind this old rule is that buildings leak air and that larger rooms or spaces have more air intake leaks than smaller ones.

Total Cubic Feet of Free Area = Total Input BTUH / 1000 x 50

Watch out: this formula may not adequately consider the reduction in volume of the room or open space attributed to contents, storage, etc. and it certainly does not adjust for modern tigh building construction. COMBUSTION AIR for TIGHT BUILDINGS explains how to provide outside combustion air for tight buildings.

Some writers simplify the formula to express this rule of thumb as

Total Cubic Feet of Free Area = Total Input BTUH x .05

Example: if we have a 180,000 Input BTUH boiler and a 40,000 input BTUH water heater installed in an enclosed utility room, how many cubic feet of space in that room would make us think we had adequate combustion air?

(100,000 + 40,000) / 1000 x 50

140,000 BTUH / 1000 = 140

140 x 50 = 7,000 cubic feet.

If our room is smaller than 7,000 cubic feet we probably don't have adequate combustion air (unless an outside combustion air source is also provided).

Calculate the total cubic feet of space in a room by multiplying the room width x room length x room height

Example: if the heating equipment is installed in an open basement that is 40 ft. x 20 ft. x 8 ft. high, we have

Cubic Feet = 40 x 20 x 80

Cubic Feet = 6400 - this basement may not provide adequate combustion air for the example input BTUH total given above.

How to Convert Round Opening Diameters (say an air duct cross section) to Opening Size Equivalent

If we are using smooth-walled round ducts to bring combustion air into the space where it is needed, and before considering the restrictions on air flow caused by duct bends and length (friction losses) we start by simply calculating the cross-sectional area of the duct:

Pi r2 = the area of a circle or the cross-sectional area of a round combustion air supply duct

Pi (also written as Π) = 3.1416 - a constant

r = the radius = half of the diameter of the circule

We can use any unit (cm, inches, feet) as long as we stick to the same unit through.

Example: a 6-inch diameter round air duct has a cross section (or area) of

Area = 3.1416 x (6 / 2)2 inches

Area = 3.1416 x (3)2 inches

Area = 3.1416 x (3 x 3)

Area = 28 of space - which, if unrestricted by duct length, bends, or screens, and if we use our first rule of thumb (one inch per 1000 BTUH) would support about 28,000 Input BTUH

Watch out: square duct area is not equal to round duct area in air flow capacity. That's because air flowing through a square or rectangular duct (or chimney) does not flow uniformly - the area of the corners of the rectangle moves less air. For you who left your calculator at home and left Pi in the refrigerator, here is the square inches of cross section opening size for common round duct diameters:

Table of Combustion Air Suply Duct Sizes vs. Input BTUh for Natural Draft Heating Appliances

Round Duct Diameter
in Inches
Duct Cross Section Square Inches
(Round Opening Size)
Approximate Total Input BTUH Supported at This Combustion Air Duct-Vent Opening Diameter2
84,800 2
115,000 2
151,000 2
191,000 2
236,000 2
339,000 2
462,000 2


1. This data is for round ducts and smooth metal duct sides; lengths of flex duct with ribbed or corrugated sides restricts air flow and will not provide as much equivalent air flow. Reference: "Evaluating Duct Work, How to Evaluate Furnace Duct Work & Cure Short Cycling or Inadequate Ductwork Problems" Vermont Department for Children and Families, Office of Economic Opportunity, -‎ retrieved 12/5/2013

2. Really? We need further research on these figures. they significantly exceed the 1000 BTUH per square inch of area. Citation: David Clark, Home Inspection Newsletter, retrieved 12/5/2013

3. These numbers are for round opening sizes used to provide combustion air to heating equipment.

See SUPPLY DUCTS & REGISTERS for details about HVAC heating or cooling supply & return air duct sizing, air flow, and for matching HVAC air duct sizes to equipment BTUH rate or heating capacity.


Combine Combustion Air Sources to Check the Combustion Air Requirements

When room volume in cubic feet is inadequate to provide safe combustion air we can add combustion air by providing an outside air source.

Combustion Air Requirements Specifictions for Power Burners

For combustion air requirements for power burner fired heating appliances see research by Utiskul (2012) cited in the REFERENCES for this article. Utiskul, writing for the NFPA discusses NFPA 54 cited by U.S. heating appliance manufacturers for combstion air standards, and in Canada, CSA B149.1-10 (which gives different criteria). Excerpting notes on combustion air for power burners the author notes:

NFPA 54 requires the largest outdoor openings for combustion air in comparison to the other referenced guidelines. The opening free area required by NFPA 54 ranges from 0.33 to 1 square inch per kBtu/hr input rating, depending on the number of the openings and how they communicate with the outdoors. The other guidelines require combustion air openings with free area ranging from 0.08 to 0.14 square inches per kBtu/hr, which is consistent with providing an additional approximately 30% air supply to the combustion process. CSA B149.1-10 contains a separate provision for appliances equipped with power burners to supply combustion air with outdoor openings sized to 0.03 square inches per kBtu/hr of the total burner input rating.

Power burner boilers have the highest range of energy input ratings (60,000 to 83,600,000 Btu/hr) in comparison to that of water heaters (60,000 to 40,300,000 Btu/hr) and furnaces (30,000 to 9,800,000 Btu/hr). A majority of the manufacturers (60%) included in this study reference NFPA 54 for combustion air requirements (0.33 to 1 square inches per kBtu/hr input) for their power burner appliances with energy inputs ranging from 30 to 92,000 kBtu/hr. Of the 25 manufacturers, eight recommend that combustion air be provided by openings sized to 0.03 to 0.75 square inches per kBtu/hr of the total appliance input rating from 150 to 83,600 kBtu/hr. The opening size in accordance January 30, 2012 QMS QA ID No. 1103894.000 B0F0 0112 YU01 50 with these eight manufacturers is approximately 25% to 97% smaller than that of the NFPA 54 requirements. The remaining manufacturers recommend combustion air requirements based on a specified volumetric flow rate from 0.24 to 0.47 cfm per kBtu/hr of appliance input rating.  

Based on the review of available literature data, a review of the manufacturer’s requirements for combustion air, the investigation of the combustion air required for power burners, and the theoretical analysis of air flows through openings, sizing criteria for combustion air openings for power burner appliances are theorized as follows:

 A minimum opening area of 0.2 square inches per kBtu/hr input rating for power burner appliances equipped with a draft control device; and

 A minimum opening area of 0.1 square inches per kBtu/hr input rating for power burner appliances that require no dilution of flue gases.

Based on the theoretical analysis provided in this study, these theorized combustion air requirements should provide an adequate amount of combustion air for proper appliance operation and will optimize overall building efficiency by reducing unnecessary area in openings. It is strongly recommended the theorized sizing criteria be validated through fullscale field experiments, which will provide a basis for new code development. - Utiskul (2012)

Guide to a Simple Combustion Air Safety Check for Gas Fired Heating Appliances

Thanks to Tjernlund Products who recommended this procedure, we describe a simple combustion air safety check that can be performed by a homeowner or by a heating service technician.

This procedure is only intended for buildings where LP gas or natural gas heating appliances (heating boiler, warm air furnace, water heater) are installed AND where a flue gas spillage safety switch such as a Tjernlund UC1 Universal Control, MAC1E or MAC4E auxiliary controls for gas fired equipment or Field Controls Gas Spillage Sensing Kit Model GSK-3, GSK-4, GSK-250M switches are installed on those appliances. (Similar products are provided by other manufacturers.)

  1. Close all doors and windows of the building. If the gas fired heating appliance is installed in a utility room or closet, close the entry door to that room. Close all fireplace dampers. Turn on the clothes dryer (if installed) and all exhaust fans such as range hoods, bathroom exhaust vent fans, whole house fans, and radon mitigation fans, running all fans at their maximum speed.
  2. Turn on and place in operation the gas fired heating appliance on which the flue gas spillage sensor switch to be tested has been installed. Set the appliance thermostat for continuous operation. If other gas fired appliances are installed in the same building run those appliances at the same time.
  3. Allow the fans turned on in step 1 to operate for fifteen minutes.
  4. If the flue gas sensor spill switch trips during the fifteen minute test period this means that an unsafe condition exists. Check all of the appliances for a venting malfunction and check for adequate combustion air. (The occurrence of the flue gas spillage safety switch tripping may indicate inadequate combustion air but it could also indicate an unsafe chimney or some other unsafe condition.)
  5. TURN OFF FUEL SUPPLY to the gas fired appliance that caused the safety switch to trip.
  7. Return all windows, doors, fans, to their previous conditions of use.

SAFETY WARNING: If AT ANY TIME your gas fired equipment has shut down in SAFETY OFF position it may be due to a resettable flue gas spill sensor switch. Check with your heating service company - you might think you can avoid a costly heating service call, BUT BEWARE: because flue gas spillage is very dangerous, including the production of potentially fatal carbon monoxide gas, don't simply reset the system without finding out what caused the problem in the first place.

MORE SAFETY WARNINGS: in addition to our safety warning above, Tjernlund explains that flue gas safety switches are intended to alert the building occupants to a potentially dangerous condition.

But flue gas spillage safety switches are not a substitute for a regular chimney safety inspection nor do they replace regular heating appliance inspection and maintenance by a trained technician. Those steps must be taken as well.

What causes flue gas spillage:

Please see SPILL SWITCH, FLUE GAS DETECTOR for a discussion of the causes of flue gas spillage in buildings and for a description that contrasts inadequate combustion air with other causes of dangerous flue gas leakage into buildings.

Bachrach Corporation, a manufacturer of heating system test equipment opines that gas fired equipment is more likely to have flue gas spillage from a blocked chimney than from building depressurization due to inadequate combustion air supply. We're not sure what data supports that view.

Certainly home inspectors find many heating appliances installed in tiny closets with no outside combustion air and a door that, when shut, blocks off air to the appliance. We have also observed that gas fired heating equipment operated just fine in a building until a new owner installed a whole house ventilation fan system.

Combustion Air Defects & Safety Hazards at Mobile Home Heating System

Below our photographs illustrate several unsafe conditions at a house trailer's heating system. There are no return air flow from the conditioned (occupied) space to the furnace warm air intake, questions about adequate combustion air, and dirt blocked air flow passages.

Mobile home furnace (C) Daniel Friedman Mobile home furnace (C) Daniel Friedman
  • No return air from occupied space to the furnace: Above we notice that when the door to the "furnace closet" in this mobile home is shut, there is no return air movement from the home's heated interior to the furnace - creating the most-expensive possible way to operate the furnace. We call this a "one way" heating system: scrounge some air from a cold outdoor or crawl space source, heat it, and blow it into the living space.
Mobile home furnace (C) Daniel Friedman

In addition to the absence of return air to the heating furnace we notice that

  • Dirt blockage of air flow: the return air inlet grille is partly blocked by dust and debris, further reducing air flow, increasing heating cost, and ...
  • Watch out: there is an increased risk of potentially fatal carbon monoxide poisoning if the system lacks adequate combustion air
  • Blocked air filter: if an air filter is installed (remains to be discovered), given how dirty is the exterior of this furnace the air filter may be equally dirty and airflow blocked
  • The leak stains and corrosion on and below the heating flue indicate that the chimney and flue are leaking, risking hidden damage, leaks, holes, water or rust damage to the furnace heat exchanger: all further pointers to an unsafe heating system
  • On some of these installations combustion air is provided from outdoors through wall or floor openings in the furnace closet, increasing safety but ignoring operating cost
  • Unsafe chimney outside: incomplete, leaks, missing cap, too short, bad draft - are illustrated by our outdoor heating flue/chimney photos earlier on this page.

Combustion Air for Woodstoves - How is It Provided?

The link to the original Q&A article in PDF form immediately below is followed by an expanded/updated online version of this article.

  • Q&A on Outside Combustion Air for Woodstoves- PDF version, use your browser's back button to return to this page. Original article, Solar Age Magazine, November, 1985, adapted and updated for December 2010.

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

Question: how do I provide combustion air for a woodstove?

I've often read about the necessity of providing combustion air to a woodstove, but there's usually not much explanation of how it's done. Is it just a pipe that ends near the stove inlet? - Chris Rich, New Castle VA

Answer: hard-ducted outside combustion air required for some woodstoves - in manufactured housing

Many wood-stove manufacturers provide a special line of stoves intended for manufactured housing. These woodstoves must have an external combustion-air inlet hard-ducted directly into the stove.

Often the combustion air supply duct comes up through the floor under the woodstove (or coal stove or airtight wood-burning fireplace), making it almost invisible from inside the room where the woodstove is located.

This makes it attractive for the homeowner, and easy for the builder to install.

A Guide to Combustion-Air-Related Heating Equipment Malfunctions & Their Implications

  • Noises & soot buildup can lead to a potentially dangerous puffback which can damage the heating equipment and blow soot and smoke throughout the building.
  • Lack of adequate combustion air can result in improper system operation, sooting, loss of heat, noises, smoke, and potentially, the production of carbon monoxide or other flue or combustion gases which escape into the building - potentially dangerous



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